1. Field of the Invention
The present invention relates to a developer sheet for forming high density images. More particularly, the developer sheet contains a developer material coated in solution form on its upper surface which includes a particulate reactive material which is capable of reacting with a substantially colorless chromogenic material upon contact to generate an image, a binder material and a viscosity increasing additive for increasing the viscosity of the developer layer solution after coating
2. Description of the Prior Art
Transfer carbonless copy systems and transfer imaging systems are well known in the art. These systems comprise two sheets which are used to form a visible image. The first sheet, the transfer or donor sheet, typically contains on one of its surfaces, a colorless color-forming agent. Often, the color-forming agent is contained in pressure rupturable microcapsules. The second sheet, the developer or receiver sheet, typically is a substrate having a layer of a developer material coated in solution form onto its front surface which can react with the colorless color-forming agent to form a visible image. In practice, to produce an image, the two sheets are aligned so that the colorless color-forming agent faces the developer material. Upon the application of pressure, the colorless color-forming agent reacts with the developer material on the developer sheet to form a visible image.
For a particular technical application, photosensitive imaging systems employing microencapsulated radiation sensitive compositions which are the subject of commonly assigned U.S. Pat. No. 4,399,209 to The Mead Corporation as well as copending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982 (corresponding to U.K. 2,113,860) have been developed. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. The image-wise exposure hardens the internal phase of the exposed microcapsules. Following exposure, the imaging sheet is assembled with a developer sheet and the assembly is subjected to a uniform rupturing force by passing the sheets through the nip between a pair of pressure rollers. Upon passing through the pressure rollers, the microcapsules image-wise rupture and release their internal phase whereupon the color former migrates to the developer sheet and forms an image. The imaging system can be designed to produce monochromatic or polychromatic full color images.
Although this system has proven to be quite successful, minor drawbacks have occurred with respect to the developer sheet. More specifically, to produce the developer sheet, a developer layer, which includes a reactive developer material, typically a phenol-formaldehyde novolak resin, and a binder material is dispersed in a solvent, typically water, and the dispersion is coated onto a support. The support is thereafter heated to drive off the water, leaving a developer layer of reactive particles and binder.
When a visible image is produced by the reaction of a colorless chromogenic material with the particulate developer layer, it is important that the number and size of the pores located between the respective reactive particles of the developer layer be maintained at a maximum. According to a capillary model for oil transfer, the density of the image produced is proportional to the square root of the effective capillary radius of the developer material. Accordingly, any reduction of the number of pores and/or effective capillary radius pore size reduces the image density.
Studies have shown that during the coating and drying steps necessary to form the developer sheet, a significant problem occurs in that the binder material, which is typically a water-soluble binder such as polyvinyl alcohol, migrates to the top surface of the developer layer. As a result, the binder material can partially block the capillary pores located between the reactive particulate developer material. Thus, as the number and effective size of the capillary pore radius is reduced due to the binder migration, high density images are difficult, if not impossible to produce.
In transfer systems, two types of materials are commonly used as developer sheets. The first material is a developer coated paper and the second material is a developer coated polymeric substrate, such as polyethylene terephthalate. The end product of the latter receiver sheet is typically an overhead transparency. The above described binder migration problem is particularly pronounced in developer sheets of the second tYpe (i.e., polymeric substrates). It is hypothesized that due to the porous nature of paper, when the developer material is dried after coating onto the paper web, the water may be removed from either the top or bottom surface of the paper substrate. Accordingly, for the water removed from the bottom of the substrate, binder migration does not pose a particularly serious problem, as the bottom surface of the substrate does not contact the chromogenic material. By comparison, in the case of a polymeric substrate, evaporation of the water upon drying can only occur from the top surface of the substrate. Accordingly, any binder migration which occurs upon drying is directed towards the top of the substrate, thereby reducing the effective capillary pore size between developer particles. Thus, when desiring to produce high density images on polymeric substrates, solutions to the problem of binder migration are of particular importance.
Attempts have been made in the art to improve image density utilizing transfer systems. For example, in a nonphotosensitive imaging system, U.S. Pat. No. 4,578,690 assigned to Nashua Corporation discloses that a superior image is produced where a developer material is utilized which includes a phenol-formaldehyde condensate oligomer mixture, an aromatic carboxylic acid and an inorganic magnesium compound, in addition to conventionally used constituents such as binders, whitening agents and fillers. The reference further discloses that the viscosity of the composition can be reduced by the addition of phosphate materials, such as hexametaphosphate.
U.S. Pat. No. 4,115,327 assigned to Sumitomo Durez discloses improved aqueous color developing compositions for copying papers. The developing compositions are produced by condensing a reaction mixture including an aldehyde and a phenol to produce a novolak precondensate, emulsifying the resin condensate by agitation in the presence of an emulsifier, and treating the emulsion with wet pulverizing means. The reference further discloses that the resin may include starches, surface active agents, and protective colloid substances including, for example, polyvinyl alcohols, polyacrylamides, hydroxyethyl cellulose, methylcellulose, carboxymethyl cellulose, gum arabic, gelatin, sodium alginate, casein, polyvinyl pyrrolidone, styrene-maleic anhydride copolymers and polyacrylic acid. The reference does not consider the problem of binder migration as a result of coating the developer emulsion onto a substrate.
One solution proposed in the art to reduce the effect of binder migration on a coated paper is to simultaneously apply a vacuum to the opposite side of the coated web during the drying step. See, for example, Shelendich, J., "High-rate Drying of Aqueous Air-knife Coatings", Tappi, 55, No. 3, pp. 420-3 (1972). This solution is unavailable when utilizing polymeric substrates.
In a non-carbonless and a non-photosensitive imaging system technical application, it has been suggested to add a viscosity enhancing modifier to the aqueous phase of a paper coating. For example, Fairchild et al., in their article "Controlling Migration in Paper and Board Coatings", Invest. Tec. Paper 5, No. 15, pp. 53-72 (1968), suggest the addition of sodium alginate to aqueous starch containing coatings which are used to coat paper and board materials.
Thus, there exists a need in the art for developer sheets which are capable of producing high density images, and particularly for developer sheets which are used in photographic imaging systems.